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New Scientist Live

Plasma tidal wave may tell us if black holes destroy information

Shedding light on black holes

ALFRED PASIEKA/SCIENCE PHOTO LIBRARY

By Jennifer Ouellette

A laser-driven tidal wave could test a question that has long plagued physics: is the information inside a black hole lost forever or somehow preserved through the mysterious machinations of quantum mechanics?

The defining feature of a black hole is thought to be that anything that crosses the event horizon – the proverbial point of no return – can never escape and is lost forever.

But in the 1970s, Stephen Hawking discovered that black holes aren’t truly black. If a virtual particle pair pops into existence near the event horizon, and one falls in, the black hole must lose a tiny bit of mass in the form of energy. So black holes will radiate tiny amounts of energy – dubbed Hawking radiation – and evaporate over time. The bigger the black hole, the longer it takes to evaporate.

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So what happens to everything that has fallen into the black hole? Logic dictates it, too, should be lost. But quantum mechanics holds that information must be conserved and cannot be lost – hence the paradox.

Unfortunately, there’s no good way to study an actual black hole up close to test what’s really going on. So physicists have been exploring “analogue” black holes that mathematically mimic their celestial counterparts.

One possibility is that the information is preserved via entangled photons, which share a quantum relationship with each other no matter how distant they are, and is released in a burst of energy as the black hole winks out of existence. If physicists could find correlations between the original escaped partner and a photon re-emitted as radiation, this would be strong evidence that information is indeed conserved.

Mirror, mirror

Researchers have suggested that an accelerated mirror could mimic a black hole’s event horizon, giving physicists a way to look for these correlations in the lab. Photons reflected back from the mirror would represent the Hawking radiation, and photons trapped at the moving mirror boundary would be the abandoned partners. When the mirror stops moving, it should create a sudden burst of energy, similar to the death throes of a black hole.

These accelerators work by shooting pulses of intense laser light into plasma to create a wave rippling through the cloud of ionised gas, leaving a wake of electrons akin to those that form behind a speedboat in water. As more electrons are pumped into the system, they draw energy from surfing that wake and accelerate, building in intensity like a tsunami.

“In order to create such plasma ‘wakefields’, the laser must dump its energy into the plasma,” says Chen. “By the law of conservation of energy, the laser pulse as well as its wakefield must therefore both slow down.”

To counter this tendency, Chen and Mourou devised a way to accelerate the plasma wakefield itself, which can be thought of as a plasma mirror. This can be done, they demonstrate, by tailoring a plasma in such a way that its density increases gradually.

Chen and Mourou have yet to build such an experiment, but they believe it can be done with existing technology. The setup could also be used to model other properties of black hole, such as how it distorts space-time.